Diagnosing and Reparing Commodore 64s

OK, but be warned you might not like my answers :-). Err, yes... Been therem, done that (but not with C64s...). Whether something is worth repairing is up to you. It may well not make financial sense to repair a C64, given how commn they are and how cheap they tenet to me, but the fiannacial rason is not the only reason for doing something (in fact to me it's rarely a reason at all). If you want otlearn how to repair things, it's not a bad machine to start on. And you will learn a lot. You might even come to ejoy it. THose seem to me to be reasons why it is worth repariign a C64. Much of what follows is not going to be C64-specific. That;s not suprising, IMHO almost all computers -- darn it, all electronic devices -- are diagnosed and repaired in much the ame way, There are 2 parts ot the repair. First you must find out what is wrong, second you put it right. Notice the order there. It is, IMHO, very foolish to start trying to reapri things, replace parts, etc until you have a lcear idea as to what is wrong amd what therefore needs replacing. Since your questions are in the opposite order, I will anseer them like that, though. My view (having strugled with them) is that the non-tempearture-controlled hobbyist irons, things like the 25W ones that I guess Radio Shack sells, are not worth trying to use. You will have lots of problems and you will get frustrated fast. A tempaerature-cotnroled iron is the thing to go for. I use a Weller TCP, about 50W. A non-controlled 50W iron would get hot enough to damage the board and components, bnt this has a thermostat built-in to keep the temperature constant(ish). And being a 50W iron it has enoguh power to heat up large bits of metal, like PCB ground planes, screenign cans, etc. Others swear by electronic-controlled irons, still others by RF-heated irons, etc. They are nice, but very expensive. Unless you are sondiering buying a professioanl desoldering station (and I doubt it), but one of those little spring-loaded pumps. Make sure you can get repalcement (ptfe) tips for it, you will need them from time to time. You will ahve to practice using it. A couple of quick tips. Firstly, if you are sure a component is bad, cut off the pins, remvoe the body of the componet, and despdler the pins one at a time. Often you can just melt the sodler and yank the pin out with pliers. To claer the holes in the PCB (sometimes needed anyway), it's easiest ot melt the soder with the iron on one side of the PCB and such from the other side. If A pin won't desolder cleanly, resodler it (use soem new solder) and try again. Often that does it. As regards RSI. I once upgraded the memory board in an HP Integral. The board I had was half-populakted with RAM chips, but it had (of course) been wave-soldered at the factory, so all the spare holes were blocked. I had to clar out 256 holes (16 RAM chips at 16 pins each) plus those for decoupling capacitors etc, My hand ached a bit afterwards, but no harm done... OK... Let's take a step back. The wway I repair things -- the way I learnt to repair things many years ago -- goes like this : 1) Work out what the thing _should_ be doing 2) Find out what it actually is doing 3) Compare the two 4) Figure out what could case the observed behaviour to differ from the expected behaviour, that's the fault. If necessary do more tests to find out exactly what is wrong. Now let's look at some of those steps. The scheamtic sare useful to work out wwhat it should be doing. Somebody with experience can look at a schemaitc and have a good idea as to what the signals, voltages, etc should be on most points in the circuit. Yes, that is a big step for a beginner, but most service manaulas were not written fro beginners. When you have more experience you will come to realise that often all you really need is the schematic. Of coruse sometimes other have been helpful and noted down foltages (e.g. power supply otuputs voltages), signals 9e.g. master clock signals, video timing signals) so you don't have to work them allout for yourself. The next problem is that you cna't useuflly detect electricity yourself. So you need measuring insturments to do that. These will tell you what hte circuit is actually doing, step (2) of my procedure. There are many bits of test equipment aviaalble, some general-purpose (used with jus abotu all electrical/electornic devices), some very specialised. Let's stick to the general-purpose ones. A multimeter measures voltage, current or resistnace (if you are not familiar with those terms then you really need to start by going back a few stanges and learnign some basic electroncis before you start fixing computers). It's by far the most used test instrument. You se it to check power supply voltages (a very common problem), to test swithces, connectors, etc. It's not so useful for fiding logic fualts. An oscilloscope (oftne shortened to 'scope) is a device that displays a graph of votlage (vertial) agaisnt time (horizontal) (OK, I know there are many otehr thigns it can be used for, but let's keep things simple for now). The simpler 'scopes ahve no intenral storage and are really only useful on repetitive waveforms. For example the AC ouptut form a mains transformer, the signals in a swithcing regulator (but take _great_ care if you start workign on these!), video timing signals, etc. More advanced/modern 'scopes have storage. You can get them to record voltae agaisn time for pwehaps 10us and then display the grpah for evermore. So you could look at the continuosuly clangin signals on a computer bus. a logic probe is a farily simple insrument that displays on LEDs wheter a signal is a logic 1, a logic 0 or changing. Althoguh it is simple, it's suprisingly useful. FOr example, if a comuputer is running a program, all the data bus lines are changing as the instructions are red by the processor. So touchign the logic probe onto each of them in turn should show that on the probe's LEDs. If one signal is always high or low you know you have a fault there. An instruemnt you haven;'t mentioned and which I find vey useful is a logic analuser. This is cross between a logic probe and a storage 'scope. It samples nad records a number of logic signals and displays them. OK, it doesn't record the actuall voltages, only whether they are 0 or 1, but it will orten record faster than a cheap storage 'scope and it will record more signals (32 channel logic analusers are commmon, 32 channel storage 'scopes are not). Some years ago HP made a pocket-sized logic analyser that they called an 'advanded logic probe'. It went under the name LogicDart. It was as easy to sue as a lotgic probe, but told you rather more about the devic eunder test. They are not easy to find now, but I find mine (bought when it just came out) very useful. I don;t find things like diagnostic programs or cartridges to be useful. The point is, they are running on a piece of defective hardware so (a) there will always be faults they can't find (a diagnostic program is not goign to be able ot find PSU problems in a C64, for example) nad they might well give misleading reuslts because the hardware is malfunctioning. They have their uses for finding problems in non-essential areas fo the machine (for exampel if a C64 ahs sound problems, you might have a program to put the right vaulse in the SID chip to prosduce a known output), but not for intiial troubleshooting. -tony